One problem with diesel engines is that the exhaust gas contains particulate matter (hereinafter may be referred to as “PM”) that is composed mainly of nitrogen oxides (NOx) and carbon and causes environmental pollution. In one general method of removing PM that causes the problem, a diesel particulate filter (DPF) formed of a porous ceramic material is disposed in an exhaust gas passage to trap PM. Although the PM is accumulated in the DPF, the trapped PM is generally subjected to intermittent or continuous combustion treatment to remove the PM so that the DPF is regenerated to its original state before trapping of the PM.
General examples of the DPF regeneration treatment include a method in which PM is burned by forced heating from the outside using an electric heater, burner, and the like, and a method in which an oxidation catalyst is placed closer to the engine side than the DPF to convert NO contained in the exhaust gas to NO2 so that the oxidizing ability of NO2 is used to burn PM.
However, to use the electric heater or burner, an external power source must be provided. In addition, for example, a mechanism for actuating the electric heater etc. when the need for regeneration of the DPF arises must be provided separately. Accordingly, the exhaust gas purification system itself is complicated. When an oxidation catalyst is used, the combustion of PM may be insufficient in some cases because the temperature of the exhaust gas is not high enough for the catalyst to exhibit sufficient catalytic activity and because a sufficient amount of NO necessary for the combustion of PM can be obtained only under certain operating conditions.
A more preferred DPF regeneration treatment method that is being contemplated is to burn PM with its combustion start temperature reduced by the catalytic action of a catalyst supported on the DPF itself. The most preferred method is to burn the PM continuously at the temperature of the exhaust gas, and this is the ultimate goal.
At present, Pt, a catalyst metal, supported on high-specific surface area alumina or the like is used as an oxidation catalyst (PM combustion catalyst) for burning and removing PM trapped by a DPF. However, the catalytic action of Pt on the combustion of PM is low at the temperature level of is the exhaust gas from an engine. Therefore, may be difficult to burn PM continuously using the heat of the exhaust gas. Accordingly, means for forced heating from the outside must also be used. In addition, the price of Pt varies with the balance between supply and demand, and this can cause a problem of large fluctuations in cost.
A PM combustion catalyst may undergo a rapid temperature increase due to the heat generated by combustion of PM. Therefore, the PM combustion catalyst must have properties such that a reduction in catalytic performance (thermal degradation) after thermal history at high temperatures is as low as possible.
Patent Documents 1 to 3 disclose, as oxidation catalysts including a composite oxide of ceria as a base material and containing no noble metal element such as Pt, mixtures containing Ce, Bi, and an optional transition metal element.    Patent Document 1: Japanese Patent Application Laid-Open No. Hei 6-211525    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-238159    Patent Document 3: Japanese Patent Application Laid-Open No. 2006-224032